I know you didn't ask for how but when; I think the two kinda go hand in hand. As usual on the internet, YMMV:
As previously noted, you need to know your last frost date - for frost-sensitive types (plants, not people). Cold-hardy plants can tolerate some cold temps, esp. if you provide some protection, cover before temps get too cold. I had one tomato plant survive a hard spring frost by putting a hot-water cozy under cover with it (boiling hot water, couple layers of protection & plastic bag over all). The plant had a touch of frost damage; note I don't recommend this for normal use, I just wondered what would happen and know I (& you) know. Maybe a few degrees colder would've wiped the plant out. The water bottle set out at 7-8 PM, was cold in the AM :) One garden-way book I used to have said if you don't lose some plants on both ends of the growing season, you aren't starting soon enough and you're quitting too early!
On to your question; given any plant, there is a number of days to harvest, usually this date is from TP or for direct sowing. Recommended start date for leeks and onions from seed is in February! Naturally you will need some good light; if you don't have a dirt-tolerant spouse, good southern exposure or have pets, I would recommend a small area kitted out w/flourescent lighting (rel. cheap, minimal fire hazard
- don't pour water over them). These lights need to be CLOSE to your seedlings, and you'll want to rotate the seedlings and swap middle-to-end plants every few days.
Use GOOD potting soil. If you can get it, use a professional mix, not the stuff sold by the box stores. Go to a nursery. It's a little more, but worth it. One thing a seedling doesn't need is fertilizer-fortified potting soil. A seed contains all the fuel a seedling needs to get started, after that it will need water and light.
In general, I find that it's important to figure on transplanting seedlings before they get pot-bound. So you don't want to start too early :) And you'll need some hardening-off days; set tender plants outside for a short time, not too much wind, protected from direct sun, lengthen time gradually. Some plants are more sensitive than others, and wind is as damaging as pests (I have a convenient stone ledge - except when there;s wind!)
You can (generally) always pot a plant on when it starts to outgrow your pots; usually seedlings get stronger after this. Some plants that are not recommended for TP do just fine for me; I have a tough time using those bio-degradeable peat pots, which every year I break down and try, really don't do well - no comparison. There must be a market for these things though, victory garden swears by them. You will of course need to take care when handling tender seedlings. Handle by a leaf, not the stem - you might accidentally crush the stem. The downside to potting on? You'll need more space, more light and you'll have to balance between a growing plant and the weather. You can retard growth SOME by withholding fertilizer, water but it gets tricky to do w/out stressing the seedling too much. If there's still snow outside, then you'll know you started too soon - I'd start over :)
Good luck!